Re-Thinking The Antimatter Rocket

by Paul Gilster on April 2, 2012

Once when reading Boswell’s monumental life of the 18th Century writer and conversationalist Samuel Johnson, I commented to a friend how surprised I had been to discover that Johnson didn’t spend much time reading in his later years. “He didn’t need a lot of time,” replied my friend, a classics professor. “He tore the heart out of books.” That phrase stuck with me over the years and re-surfaced when I started working with Adam Crowl. More than anyone I know, Adam can get to the heart of a scientific paper and explain its pros and cons while someone like myself is still working through the introduction. And because of his fine work with Project Icarus, I thought Adam would be just the person to explain the latest thinking about a classic concept that Friedwardt Winterberg would like to take to the next level.

by Adam Crowl

In Jules Verne’s From the Earth to the Moon, the bold Frenchman Michel Ardan, in his first speech to the Baltimore Gun Club, when discussing travelling to the Moon via a cannon-shell, makes the following statement…

Well, the projectile is the vehicle of the future, and the planets themselves are nothing else! Now some of you, gentlemen, may imagine that the velocity we propose to impart to it is extravagant. It is nothing of the kind. All the stars exceed it in rapidity, and the earth herself is at this moment carrying us round the sun at three times as rapid a rate… Is it not evident, then, I ask you, that there will some day appear velocities far greater than these, of which light or electricity will probably be the mechanical agent?

Rockets replaced cannon-shells as the preferred means of interplanetary travel in the early 20th Century, thanks to the work of Tsiolkovksy, Goddard, Oberth and Noordung. They took up Verne’s insight and developed Ardan’s hand-waving further. Applying electricity to rocket motion resulted in the Ion Rocket, and applying light, the Photon Rocket. However the first rocket scientist to propose an engineering solution to how light might be directly harnessed to rocket propulsion, rather than just pushing solar-sails, was Eugen Sänger [1].

Antimatter and the Photon Rocket

Sänger’s discussion of photon rockets showed clearly how difficult it would be – every newton of thrust would require 300 megawatts of photon energy released. Any vehicle generating photons by conventional means would be confined to painfully low accelerations, thus Sänger proposed using matter-antimatter reactions, specifically the mutual annihilation of electrons and positrons, with the resulting gamma-rays (each 0.511 MeV) being reflected by an electron-gas. Unfortunately the electron-gas mirror would need a ridiculously high density, seen only in white-dwarf stars.

The next stage for the matter-antimatter photon rocket saw the work of Robert Forward [2], and more recently Robert Frisbee [3], who applied more modern knowledge of particle physics to the task. Instead of instant and total annihilation of proton-antiproton mixtures, resulting in an explosion of pure high-energy gamma-rays in all directions, the reactions instead produce for a brief time charged fragments of protons, dubbed pions, which can be directed via a magnetic field. According to theoretical analyses by Giovanni Vulpetti [4], in the 1980s, and more recently by Shawn Westmoreland [5], the theoretical top performance of a pion rocket is a specific impulse equivalent to 0.58c. However the pion rocket isn’t strictly a pure photon rocket and suffers from the inefficiency of magnetic nozzles. Simulations by John Callas [6] at JPL, in the late 1980s suggested an effective exhaust velocity of ~1/3 the speed of light could be achieved.

The other difficulty of matter-antimatter propulsion, as graphically illustrated by Frisbee’s work, is the extreme difficulty of storing antimatter. The old concept of storing it as plasma is presently seen as too power intensive and too low in density. Newer understanding of the stability of frozen hydrogen and its paramagnetic properties has led to the concept of magnetically levitating snowballs of anti-hydrogen at the phenomenally low 0.01 K. This should mean a near-zero vapour pressure and minimal loses to annihilation of the frozen antimatter. What it also means is immensely long and thin spacecraft designs. Frisbee’s conceptual designs are literally the size of planets, thousands of kilometres long, but merely metres wide. This minimises the gamma-radiation exposure of heat-sensitive components and maximises the exposure of radiators to the cosmic heat-sink. To achieve 0.5c, using known materials, results in vehicles massing millions of tonnes [3].

Harvesting the Fire

Friedwardt Winterberg’s recent preprint [7] suggests a different concept, with the promise of near total annihilation and near perfect collimation of a pure gamma-ray exhaust. Poul Anderson described such a vehicle’s operation in fiction in his Harvest the Fire (1995), describing an advanced matter-antimatter rocket – the exhaust was so efficiently directed that it was invisible for thousands of kilometres before finally appearing as a trail of scattered energy. So what is Winterberg proposing?

We’ve encountered Winterberg’s work before [8] in Centauri Dreams in his designs for deuterium fusion rockets, and his new work is an outgrowth of his work on the magnetic collapse of ions into incredibly dense states. Using the technique he describes, high compression of fusion plasma can be achieved, but in the case of a matter-antimatter ambiplasma (a plasma that is an even mix of the two) the result is even more spectacular.

Essentially what Winterberg describes is generating a very high electron-positron current in the ambiplasma, while leaving the protons-antiprotons with a low energy. This high current generates a magnetic field that constricts rapidly, a so-called pinch discharge, but because it is a matter-antimatter mix it can collapse to a much denser state. Near nuclear densities can be achieved, assuming near-term technical advancements to currents of 170 kA and electron-positron energies of 1 GeV. This causes intensely rapid annihilation that crowds the annihilating particles into one particular reaction pathway, directly into gamma-rays, pushing them to form a gamma-ray laser. By constricting the annihilating particles into this state a very coherent and directional beam of gamma-rays is produced, the back-reaction of which pushes against the annihilation chamber’s magnetic fields, providing thrust.

Figure 1. [from Ref.7] Gamma-ray Laser

The figure above depicts the processes involved – the magnetic field of the ambiplasma (from the electron-positron current) squeezes a linear atom of protons-antiprotons which begin annihilating, stimulating more annihilation, all in one direction from the annihilation being triggered at one end of the discharge. Thanks to the very confined channel created by the magnetic pinch, the laser beam produced has very limited spread. Intense magnetic-field recoil is created by the firing of the gamma-beam, with a pulsed field-strength of 34 tesla. The recoil force can thus be transferred back to the vehicle by the right choice of conductor surrounding the reaction chamber.

Winterberg ends his paper with an anecdote about Edward Teller, one of the many fathers of the H-bomb, who was of the opinion that photon rockets would eventually be possible – in “500 years” which equates to “impossible” in the minds of the short-sighted. Certainly making antimatter efficiently will be a Herculean task, as the energy requirements are immense. Storing it is equally “impossible”. However, as Winterberg notes, there might be a quicker pathway to confinement.

Over the last decade researchers at the University of Gothenburg, led by Leif Holmlid, have been studying exotic states of deuterium. In the past two years they have reported [9] an ultra-dense state, which has also been independently computed [10] to form inside low-mass brown-dwarf stars. This exotic quantum liquid is one million times denser than liquid deuterium and apparently a superconducting superfluid at room-temperature. Only minute amounts have been made and studied so far, but such a material could be able to sustain intense magnetic fields, up to 100,000 tesla. If it can be manufactured in large amounts, and is stable in intense magnetic fields, then the problem of magnetic confinement of anti-hydrogen at friendlier temperatures becomes more tractable.

To quote Winterberg [7], paraphrasing Teller…

Therefore, if nature is kind to us, the goal for a relativistic photon rocket might be closer than the 500 years prophesized by Teller.

Eugen Sänger is a curious story, not a reclusive as Goddard, but still a lone wolf of sorts.
Another one of those great visionaries , his lasting contribution was the Silbervogel. He and his engineer wife Irene Bredt invented the lifting body spacecraft a legacy that gave of the Space Shuttle Orbiter. (They did all the work with pencil and paper …. and slide-rules! This work was all done before 1940.)
The Reichsluftfahrtministerium was so impressed they gave him his own research facility but when Peenemünde got wind of it … Berlin was convinced to kill it as a duplication of effort. I seem to remember that von Braun offered Sänger a place at Peenemünde, but he declined*, ran a smaller research effort mostly on ramjets.

Sänger must of had ,in the back of his mind, the discovery of the positron, and must have done some calculations after the war. Finally presenting his ideas in 1953. There followed a number of papers and at least two monographs, never translated, I think. His best summary is in Handbook of Astronautical Engineering (1961) , chapter 21.4.

Here’s a question, FTL warp drives appeared in SF in the 1930’s, atomic energy for propulsion was quickly recognized both inside and outside of SF.
But I’ll be darned if I can find a reference to antimatter propulsion in SF before the 1950’s. Anybody know?

*This was similar to Frank Malina’s offer to Goddard to come and work at Cal Tech during WWII. Goddard finally wound up at Annapolis working of liquid JATO units, which did not work out. However Goddard got to do an autopsy on the first V2 delivered to the USA, how wistful that must have been!

Here’s a question, FTL warp drives appeared in SF in the 1930′s, atomic energy for propulsion was quickly recognized both inside and outside of SF. But I’ll be darned if I can find a reference to antimatter propulsion in SF before the 1950′s. Anybody know?

I don’t know yet, but a project I’m working on now ought to pull up an answer and I’ll let you know what I find. Right now, though, I’m still back in the early 20th C with the research.

Always have been a fan of matter/anti-matter propulsion, in the case of a photon drive the issue, besides the large amounts of antimatter, is the hazard of pointing the nozzle of the rocket in the incorrect direction. ie. Earth.

Mass-annihilation drives appear prominently in Heinlein – in Time for the Stars for example. But then in later 1950s he switches to fusion power for the starships (e.g. Citizen of the Galaxy) which perhaps reflects the change in understanding of particle physics that was occurring in physics at the time. Not sure I can pin-point antimatter propulsion prior to “Star Trek”.

” … Intense magnetic-field recoil is created by the firing of the gamma-beam, with a pulsed field-strength of 34 tesla. The recoil force can thus be transferred back to the vehicle by the right choice of conductor surrounding the reaction chamber.”

Okay, here you got me. I’m afraid that I don’t understand the physics here at all. I was under the impression that when the two photons were created from annihilation (positron and electron) that there would be nothing to recoil against. Is this correct that photons usually can’t be directed by magnetic fields?

Apparently the first Sci-Fi story was “Collision Orbit,” which appeared in the July 1942 issue of Astounding Science Fiction. The writer was Jack Williamson. Anti-matter was best known as contraterrene matter at that time, as coined by Vladimir Rojansk.

Good catch! Williamson got into antimatter ideas through John Campbell, Astounding’s legendary editor, and went on to write stories under his ‘Will Stewart’ pen-name that would be collected in Seetee Shock and Seetee Ship, all with some kind of antimatter connection. I didn’t know about ‘Collision Orbit,’ though — thanks!

The section on anti-matter propulsion begins on page 100. Sanger is not referenced (that I noticed) however Alfven is frequently referenced. Unlike Winterberg, JPL does not wave away the fact that about half the reaction mass is neutrinos and anti-neutrinos. They do discuss the basics of storage and reaction via an ambiplasma of matter and anti-matter, and the importance of the electrons/positrons to the ambiplasma stability which otherwise contributes little to the energy production.

Although it is 37 years old it is a very nice, concise, quantified, easy to read catalog of potential propulsion technologies.

Ah, contraterrene, have not heard that term in , well maybe 50 years!
I remember it, tho not how many occurrences there were.
Was it used as propulsion in the Williamson story?

As to lethality of the original antimatter photon rocket beam, Sänger computed this early on and concluded one could never launch one from the Earth, not even in Earth orbit… get a long long ways away before you turn it on!

Williamson’s 1942 “Collision Orbit” is not the first SF story to feature antimatter. It is, however, the first story of his well-known Seetee series about mining antimatter asteroids. I wrote about the origins of these stories in Symmetry in 2008: see also the “Logbook” sidebar. (Unfortunately the headline, “Antimatter’s Science Fiction Debut,” is not quite correct.)

The first science fiction story I have identified which uses Dirac’s ideas is the (deservedly) obscure “The Roaring Blot” by Frank Belknap Long, published in the March 1936 Astounding , but the story does not make use of annihilation. For that, we must turn to John D. Clark (known to rocketry buffs as the author of Ignition!) for “Minus Planet” in the April 1937 Astounding , where an antimatter planet on a collision course with Earth must be stopped. Antimatter featured in a couple of other stories prior to Williamson’s.

But I think the answer to Al Jackson’s question about antimatter propulsion may indeed be “Collision Orbit.” At the climax of the story (SPOILER WARNING!) grizzled asteroid miner Jim Drake, using magnets, aims an anti-iron meteor into a shaft drilled in his home asteroid, then sends a couple of iron cylinders in after it. In the fury of the resulting annihilation, a jet of vaporized material serves to deflect the asteroid’s orbit. Straight thermal rocket, no “dilithium crystals” necessary.

For a while Star Trek tried to establish that their starships could not go to warp speed until they were well out of the Sol system, but that idea obviously did not last.

Star Trek warp drives are a combination of Bussard ramjet (the glowing red domes at the front of the starship nacelles collect interstellar hydrogen) and antimatter rocket. Too bad they had to use magical dilithium crystals, which in the very earliest episodes they called lithium crystals. I did notice that the crews were much more mellow. :^)

LJK, that is the 1940 paper where Rojansky suggested that comets, and some meteors, might be composed of antimatter. Rojansky this calls “contraterrene” matter, a word he had coined in an earlier paper: “Concerning the Possibility of a Unified Interpretation of Electrons and Protons,” Physical Review 48, 108–109 (1935) :

As for Star Trek, I think (but cannot document) that using antimatter as the Enterprise‘s energy source came from Harvey Lynn of the RAND Corporation. He was the physicist who supplied Gene Roddenberry with many of the scientific underpinnings of Trek.

This causes intensely rapid annihilation that crowds the annihilating particles into one particular reaction pathway, directly into gamma-rays, pushing them to form a gamma-ray laser. By constricting the annihilating particles into this state a very coherent and directional beam of gamma-rays is produced, the back-reaction of which pushes against the annihilation chamber’s magnetic fields, providing thrust.

Might not this antimatter annihilation gamma ray laser prove to be a most deadly weapon? The whole concept of “photon drives” is basically flying around on the recoil of a super death ray. Your starship will be a death star that rides the recoil of a beam with the power to melt cities- and is left on for months or even years at a time!!

If we build such a photon rocket, we will almost certainly use it as a weapon. Should we find alternative means of propulsion like space warp generators, tachyon shunts, or other such exotic breakthroughs, annihilation gamma ray lasers may be used as deadly laser cannons. This “photon rocket” sounds a lot more like the Starship Enterprise’s phaser banks than a rocket drive.

Sanger stated in his original paper that if the photon rocket is possible, then so must be the oft-ridiculed “weapon beam” or ray gun. By the way, has anyone calculated how much damage a photon rocket’s beam could do to a city or spacecraft unfortunate enough to be situated behind the rocket when it lights up?

“By the way, has anyone calculated how much damage a photon rocket’s beam could do to a city or spacecraft unfortunate enough to be situated behind the rocket when it lights up?”

Sänger did.
A 100 ton photon propulsion ship lite up at roughly 300 km (where the ISS is) would roast the Earth! (Handbook of Astronautical Engineering (1961) , chapter 21.4.) He gives some other calculations like this too. As I mentioned before he cautions to take the thing to the other solar system before lighting it!
The prospect of an antimatter photon ship of Sänger’s has more technical problems than the Bussard ramjet… it will never be built.

And let us hope no one out there can or wants to slam a couple of neutron stars or collapsars together as a military tactic. Or can create or redirect those galactic jets like the one eminating from Messier 87.

I agree. The whole idea of a “photon rocket” is literally riding the recoil of a deadly laser cannon. Robert Heinlein really goofed when he portrayed a fleet of mass annihilation torchships taking off from somewhere out in the Pacific. He noted that the neutron backwash and heat would instantly kill anyone within hundreds of kilometers, but apparently thought that it wouldn’t matter because the ships launched out in the ocean. Later novels showed the torchships sensibly staying in orbit, where they belong, but Sänger’s calculations show that even that is not safe.

The propulsive efficiency of a photon drive must be terrible. Most rockets use much of their onboard energy accelerating the exhaust, and while some of that energy does end up in the kinetic energy of the rocket, most is lost in the exhaust. The higher the exhaust velocity, the less efficient the rocket- so mass efficient rockets are not energy efficient. A photon drive, relying as it does on the feeble pressure of light, requires an enormous power output- and most of that energy ends up the continent-melting, civilization-destroying, Kzinti- frying beam. Most of the energy of the matter-antimatter fuel must end up in the beam.

All this makes me even more interested in pursuing solutions beyond rockets. Ramjets, ion scoops, and bizarre “space drives” all offer potential solutions, some far more plausible than others!! Here are some of my personal favorite bizarre star drives-

1. Space Drives: Somehow changes the properties of space behind and ahead of the craft, creating a net thrust- a bit like those toy detergent-propelled boats science books have you make to demonstrate surface tension. Slower than light, but requires no propellent. These ideas assume there is some “structure” to space that we can push against.

2. Casimir Drive/Quantum Drive/GUT Drive/etc.: Utilizes vacuum energy, a concept that seems to be popular in the advanced propulsion field. Starship mines unlimited energy of the vacuum, allowing it to accelerate to high speeds, but limited to sub-optic velocities. Used in Arthur C. Clarke’s Songs of Distant Earth.

3. Bubble Drive: Creates a bubble of space-time around a starship that is partially “disconnected” from the rest of the universe. The bubble moves at high speed, carrying the ship with it. The ship experiences no g-forces or time dilation, and the bubble can exceed C (why is never entirely clear). The Spindizzy from the Cities in Flight operates like a bubble drive.

4. Tachyon Interconversion Drive: Ship transforms into its tachyon equivalent and zips across space at greater than C.

5. Distortion Drive/Wormhole Generator/etc. These drives modify the distance to the destination, either by “warping” space to create a shortcut or excavating some kind of wormhole. Often quite unclear on the concept. I put all FTL drives that modify distance, not velocity, in this category.

6. Teleport Drive: Teleports the ship via some kind of quantum tunneling process a shiplength across space over and over, millions of times per second. This teleportation is assumed to be instantaneous, and if the ship jumps rapidly enough it appears to exceed C. Again, I consider this one a little hazy on the details- since when was quantum tunneling assumed to be FTL?- but a neat idea nonetheless.

Obviously, most of these ideas are merely science fiction today, but so to was harnessing nuclear power or shooting a human to the Moon. There are some places to start investigating, like the Casimr effect and reported quantum FTL effects. Even if we don’t find the hoped for breakthroughs, it is a good way to explore the lingering mysteries of physics.

After the enthousiasm about the first mention of an antimatter space drive in SF, could anyone answer the question someone asked before? How do you get a recoil? And I can add: electron positron anihilation creates two photons in the opposite direction, thanks to the momentum conservation law, in the center of mass. To get two photons in almost the same direction, you need to boost the two stem particles, and the only recoil you will get comes from the boost, not the anihilation, except if the boost is enough powerfull to doppler one photon so its wavelentgh is long enough to not go through matter. In all cases, it is far from the maser description in this article.

@Chriostopher,
Do you really think warp drive is more plausible than antimatter drive? You are almost right when you describe propulsive effect of light…almost because all of this is half true, here is the magic of physic: If you see a classical space rocket going at 1000km/s that exhausts its propeller at 1000km/s then all the energy spent onboard goes in the rocket kinetic energy… but only in your frame reference. More a classical space rocket waste a lot of energy in mass energy. A light space craft that already goes very fast in your inertial frame spend much more energy in its kinetic energy than a classical space craft, more, if the space craft approach light speed, the beam is just so much doppler distorded than it has nomore energy.

Kalish, I also agree on the need for further detail, but I had taken a completely different inference from the text. Rather than decay into 2 photons, I had thought that this article implied a reaction in which the stimulated emission of the single photon produced shot backward, with other charged particles that have been created (and that are associated with that particular energy of photon??) recoiling against the magnetic field. That said, I am just as lost as you. I note that no charged particle is lighter than an electron/positron so understand how you must think this not much of an improvement.

Thanks, I should read it carefully, it is just unknown to me. It seems you need to always have a e-p plasma ready to be pushed, in an excited state. The question of the ability of the plasma to be sensitive to all the gamma rays is a question isn’t it? Because as far as I know there is no such reaction of annihilation that produce only one gamma ray in one direction. So I guess it is a global view of the process that pass over the details.

Or I missed everything.

However you can not have more impulse with annihilation than you could have with a mirror sensitive to gamma rays.

Ronan Keane (Western Reserve Academy) and Wei-Ming Zhang (Kent State University) report that the latest results from their computer simulations indicate that at least one key component of realizing a working antimatter propulsion engine — highly efficient magnetic nozzles — should be far more efficient than previously thought. And such nozzles are feasible using today’s technologies.

Paul, could you pass on my reply to “bill” April 2, “kalish” April 10 and “Rob Henry” April 11. In reading your column I had overlooked their questions. My paper has now appeared in the December 2012 issue of Acta Astronautica. It there explains that the GeV photon recoil is transmitted to the return conductor by the Mossbauer effect. It means that at the extremely large magnetic fields in the ultradense ambiplasma pinch channel, the channel in its entirety can absorb the recoil by quantized Alfven wave modes, very much as an entire crystal can do this by condensed matter phonons in the Mossbauer effect, but there for much smaller photon energies. This is a well understood quantum mechanical effect.

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In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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